This disclosure relates to the area of Uplink Power Control, UL PC. UL PC is relevant for several different Radio Access Technologies, RATs. The solutions described herein will be exemplified with regards to Long Term Evolution, LTE, wireless networks but the solutions are applicable to other types of wireless communication networks or RATs as well.
In wireless communication networks, the UL is typically a challenging link, where the available scarce energy of a wireless device, e.g. a User Equipment, UE, must be used to compensate for the losses of the channel (distance dependent pathloss, shadow fading, fast fading, etc.). Moreover, the interference produced by any UL transmissions in a multi-cell environment is also another limiting factor for the UL performance. One way to use efficiently the available energy at the UE is to control the UL transmit power. UL power control can be used on both data and control channels.
UL power control in LTE is a topic discussed earlier and part of already the first 3rd Generation Partnership Project, 3GPP, LTE standard release, i.e. Rel. 8. According to the standardised method, UL PC is typically based on compensating the pathloss to the connected cell. For example, a UE which is close to the BS will use less transmit power than a UE which is close to the cell edge. This power control principle does not take into account any impact of the selected transmit power on surrounding (or interfered) cells.
The UL power control may become even more intricate in heterogeneous networks, where different wireless access points of different downlink transmission powers are employed. The size of the respective coverage areas, also referred to as cells, for wireless access points of different downlink transmission powers may vary substantially and also the total number of UEs, the density (i.e. the number of UEs per area unit of a cell) may vary substantially. Uplink power control in such heterogeneous networks plays an important role: it balances the need for sufficient transmit power to maintain the required Quality-of-Service, QoS, against the need to control inter-cell interference and maximise the UE battery life.
In achieving this goal, an efficient power control algorithm should adapt to the characteristics of the radio propagation channel by taking into account path-loss or geometry conditions as well as overcoming interference from other users in neighbouring cells. The downlink geometry gives an indication of an experienced radio position of the wireless device with respect to the serving wireless access point and at the least one neighbouring wireless access point. The downlink geometry may be expressed as a ratio of received useful signal over received interfering signal.
In LTE, uplink power control is a combination of two terms: a basic open loop operating point for compensating for slow changes in pathloss, and a closed loop mechanism consisting of explicit control commands transmitted in the downlink for user specific power adjustments.
In single-cell configurations, the parameters that define an open loop operating point are set by utilising information related only to the serving wireless access point. The most common single-cell configurations is the pathloss-based power control method, which is also the 3rd Generation Partnership Project, 3GPP, baseline, and the load based power control method. The parameters that define the open loop operating point are set by utilizing information related only to the serving cell.
According to 3GPP, the transmit power target per resource block (PRB) for PUSCH transmission can be evaluated as PSDTX=P0+αPL+δCL, where P0 is the received power target (user or cell specific), a is the path-loss compensation factor (cell specific), PL is the downlink path-loss measured by the UE and δCL is the closed loop component.
The fractional path-loss compensation factor α is a cell-specific parameter that can be seen as a tool to control the trade-off between cell-edge data rate and total uplink capacity. Uplink power control with α=1 corresponds to full pathloss compensation. Full path-loss compensation maximizes fairness for cell-edge wireless devices by adjusting the UL power so that the received power remains constant. On the other hand, by setting α<1 a wireless device compensates only a fraction of pathloss when setting the transmit power. In this way, fractional pathloss compensation, FPC, may improve the total system capacity in the uplink by assigning relatively lower transmit power to the terminals close to the cell border (higher path-loss), so that cell-edge wireless devices cause less inter-cell interference. Typically, path-loss compensation factors around 0.8 have been shown to give a close-to-optimal uplink system capacity without degrading significantly the cell-edge data rate.
Since the parameters P0 and α determine the open loop operating point, they can be used by the operator to control the uplink power. Therefore, different choices of the parameters P0 and α can lead to different UL power control configurations.
An assumption in Fractional Power Control, FPC, is that UEs with low pathloss can increase their transmit PSD without causing too much interference. However, this assumption is only valid in homogeneous scenarios. For example, in scenarios with mixed indoor and outdoor users, increasing the power of an outdoor UE with relatively low pathloss can cause strong interference to a neighbouring indoor UE that has relatively high pathloss. In this case, it is highly possible that the indoor UE becomes power limited when it is close to the cell border and has no power to boost Signal to Interference and Noise Ratio, SINR.
An idea behind single-cell load based power control is to set the received power target in each cell, i.e. coverage area of a wireless access point, proportional to the number of wireless devices. Hence, higher uplink targets are used in cells with high load and load balancing between the cells may be achieved. Load-based power control can be combined with full and fractional pathloss power control. The UE transmit PSD target can then be expressed as PSDTX=P0+αPL+10 log10 N+δCL, where N is the number of users per cell. The number of users per cell can be directly mapped to the mean cell utilization by using the following expression
  N  =                    radio        ⁢                                  ⁢        resource        ⁢                                                  ⁢                                                ⁢        utilisation                    1        -                  radio          ⁢                                          ⁢          resource          ⁢                                          ⁢          utilisation                      +    1.  
In multi-cell configurations, the open loop operating point may be set by combining information from the serving and neighbouring wireless access points. Therefore, these schemes require more information and processing with respect to the single cell approach. Geometry-based power control (also known as interference aware power control) is the most common multi-cell configuration. In this method, the output power of each wireless device is set by taking into account the downlink gain geometry. The gain geometry is a useful measure of the amount of interference generated by a specific user and can be evaluated as G=lin(−PLBS)/Σi≠BSlin(−PLi), where PLBS is the pathloss between the wireless device and the serving wireless access point and PLi is the pathloss between the wireless device and its neighbouring wireless access point i (in linear scale). Hence, high gain geometry suggests low interference to neighbouring wireless access point, while low geometry suggests high interference to neighbouring wireless access point. After combining interference aware power control with pathloss-based power control, the transmit PSD target of each wireless device may be calculated by the following expression PSDTX=P0+αPL+kG+δCL, where k is a geometry proportional factor.
A wireless communication system employing the uplink power algorithm described above, may be efficient for determining an uplink transmission power compensating for at least a fraction of the pathloss. It is possible to determine how large a fraction of the pathloss should be compensated for by setting a corresponding value of α. Such a power algorithm may facilitate optimisation of certain characteristics in a coverage area of a wireless access point, e.g. interference situation or throughput. However, what is favourable in one coverage area of a first wireless access point may generate a bad situation in a neighbouring coverage area of a neighbouring second wireless access point.
Consequently, the overall system performance in e.g. two or three neighbouring coverage areas of corresponding wireless access points may be less than optimal if consideration is taken in each coverage area only to circumstances in that coverage area.